V. cholerae is a gram-negative bacterium that, in its wild-type state, causes severe, dehydrating and occasionally fatal diarrhea in humans. There are an estimated 5.5 million cases of cholera each year, resulting in greater than 100,000 deaths (Bull. W.H.O. 68:303–312, 1990). Over the last several decades, cholera has been considered to occur primarily in developing countries of Asia and Africa, but recently it has reached epidemic proportions in regions of South and Central America as well (Tauxe et al., J. Am. Med. Assn. 267:1388–1390, 1992; Swerdlow et al., J. Am. Med. Assn. 267:1495–1499, 1992).
Patients who recover from cholera infection have long-lasting, perhaps lifelong, immunity to reinfection (Levine et al., J. Infect. Dis. 143:818–820, 1981). The development of V. cholerae vaccines has focused on reproducing this naturally occurring immunity, but the conventional, parenteral, killed whole-cell vaccine preparation provides less than 50% protection from disease, for a duration of only 3 to 6 months (Saroso et al., Bull. W.H.O. 56:619–627, 1978; Levine et al., Microbiol. Rev. 47:510–550, 1983).
The most important virulence factor for V. cholerae in causing clinical disease is cholera toxin, a protein complex consisting of one A subunit and five B subunits. An internal deletion of the gene encoding the A subunit of cholera toxin (ctxA) in the classical strain 0395 produces a strain (0395-N1) that is highly immunogenic in humans (Mekalanos, 1983, Nature 306:551–557; Herrington, 1988, J. Exp. Med. 168:1487–1492; Mekalanos, U.S. Pat. No. 4,882,278, herein incorporated by reference).